The internal combustion engine of a vehicle transmits power to the vehicle along a transmission train comprising a succession of components. For example, in a vehicle (as shown in FIG. 1) with a front engine, rear-wheel drive, and rear axle gearbox, the front engine is connected by the clutch to a propeller shaft which terminates inside the gearbox casing at the rear axle; and two axle shafts extend from the gearbox casing, and are each integral with a respective rear drive wheel which transmits its own part of the drive torque to the road surface. This type of transmission train is an elastic-torsional system, by comprising a series of high-inertia components (e.g. the drive shaft, flywheel and gearbox) and a series of highly elastic components (the propeller shaft and wheels).
Being an elastic-torsional system, the transmission train has intrinsic oscillation modes, each of which has its own resonance frequency. More specifically, the transmission train described has three intrinsic oscillation modes: a first characterized by a node at the engine, a node at the vehicle, and an antinode at the wheels; a second characterized by a node at the wheels; and a third characterized by a node at the engine, a node at the wheels, and an antinode at the gearbox. Using real-vehicle characteristics, the resonance frequencies of the first, second, and third intrinsic oscillation mode work out at around 4 Hz, 8 Hz, and 75 Hz respectively.
An internal combustion engine has a finite number of cylinders, each of which generates a torque pulse for every two complete rotations of the drive shaft, so that the torque transmitted from the engine to the vehicle by the transmission train has a pattern varying as a function of the engine angle, and which can be modelled by superimposing a constant mean value and a series of harmonics. For example, an 8-cylinder internal combustion engine has a torque pattern as shown in FIG. 2, and harmonics of the fourth, eighth, twelfth, sixteenth . . . order, as shown in FIG. 3. The only harmonic of relatively high amplitude, however, is the fourth-order one (in an eight-cylinder engine, the amplitude of the eighth-order harmonic is roughly a quarter of that of the fourth-order harmonic). At 1000 rpm, the drive shaft has a frequency of 16.67 Hz, so that the fourth harmonic has a frequency of 66.67 Hz; at 1200 rpm, the drive shaft has a frequency of 20 Hz, so that the fourth harmonic has a frequency of 80 Hz.
When an eight-cylinder internal combustion engine goes from 1000 to 1200 rpm, the frequency of the fourth harmonic of the drive torque transmitted from the engine to the transmission train therefore increases from 66.67 Hz to 80 Hz, i.e. through the roughly 75 Hz resonance frequency of the third intrinsic oscillation mode of the transmission train. When the frequency of the drive torque fourth harmonic is in the neighbourhood of the resonance frequency of the third intrinsic oscillation mode, resonance phenomena occur, which have the antinode at the gearbox, and which generate annoying mechanical noise in the gearbox which is clearly audible by the driver of the vehicle. The reason for this is that, at around 1100 rpm, the engine is close to idling, i.e. vehicle speed is low, if not zero, so that the noise of the vehicle itself (aerodynamic noise, wheel rolling noise, engine noise) is extremely low and not enough to conceal the mechanical noise generated by resonance phenomena.
To eliminate the mechanical noise generated by resonance phenomena as described above, it has been proposed to equip the transmission train with high-torsional-elasticity members, which reduce the effects of resonance phenomena and lower the resonance frequency of the third intrinsic oscillation mode to values corresponding to below-idling engine speeds, i.e. to speeds not actually used by the engine. Such high-torsional-elasticity members may be defined by torsional dampers—which, however, often fail to provide for a sufficient reduction in the resonance frequency of the third intrinsic oscillation mode—or by a damped double flywheel of the type described in U.S. Pat. No. 5,755,143 or U.S. Pat. No. 6,306,043.
Though substantially successful in sufficiently reducing the resonance frequency of the third intrinsic oscillation mode, a damped double flywheel is expensive, bulky, and heavy, and impairs engine response, which is a major drawback in racing vehicles.